Lubricants and greases of various types are used in equipment and in manufacturing processes to reduce friction and wear and, in many situations, remove waste heat. Although some lubricants are water-based, most of the lubricants are oil-based, containing, for instance, mineral oil, poly (alpha olefin) oil, ester synthetic oil, ethylene oxide/propylene oxide synthetic oil, polyalkylene glycol synthetic oil, and silicone oil.
The main technical requirements for lubricants are that they must be able to: (a) keep surfaces of working parts separate under all loads, temperatures and speeds, thus minimizing friction and wear; (b) act as a cooling fluid removing the heat produced by friction or from external sources; (c) remain adequately stable in order to guarantee constant behavior over the forecasted useful life; (d) protect surfaces from the attack of aggressive products formed during operation; and (e) fulfill detersive and dispersive functions in order to remove residue and debris that may form during operation. The main properties of lubricants, which are usually indicated in the technical characteristics of the product, are viscosity, viscosity index, pour point, and flash point. However, more and more machinery operation environments demand an effective heat management strategy, typically requiring the use of a lubricant with a high thermal conductivity. The thermal conductivity values of the commonly used lubricating oils (without an additive) are typically in the range of 0.1 to 0.17 W/m-K at room temperature and thus they are not good heat transfer agents.
In order to meet the various requirements, one or more types of additives or property modifiers are added into the neat fluid (e.g. base oil) in a lubricant or grease composition. The neat fluid, with or without a dispersant, is herein referred to as the lubricating fluid in a lubricant or grease composition. The use of graphite particles in lubricants or greases is well known in the art. Graphite is added as a friction reducing agent, which also carries some of the load imposed on the working fluid, thereby helping to reduce surface damage to working parts. Although the thermal conductivity of graphite is much higher than that of essentially all base oils and water, few patents filed on graphite-containing lubricants specifically rely on graphite to improve the thermal conductivity of the fluid. While graphite-containing automotive engine oil was once commercialized (ARCO graphite), the potential to use graphite as a heat transfer-enhancing agent in this oil was not realized. The particle size of graphite used was typically very large, on the order of one to several microns. As a result, the graphite incorporated in the automotive engine oil had strong tendency to settle in the fluid.
Graphite particles of this size have been used to reduce friction and improve wear performance of certain fluids, e.g. in metalworking fluids. However, the use of graphite in lubricants for re-circulating systems has been decreasing, partly due to the concern that graphite could pile up in restricted flow areas in concentrated contacts, thereby leading to lubricant starvation in other areas of the system. The effect of graphite particle size on these phenomena was studied by Zhang et al who taught about utilizing nano-sized graphite particles with the mean particle size less than 500 nm to enhance thermal conductivity in fluids, but failed to disclose how these fine graphite particles performed other desired functions (e.g. wear resistance). The patent of Zhang et al. “Enhancing thermal conductivity of fluids with graphite nanoparticles and carbon nanotube,” U.S. Pat. No. 7,348,298, Mar. 25, 2008, is herein incorporated as a reference. In Zhang's patent, graphite nano particles were prepared by grinding and ball-milling carbon foam particles down to a diameter <1 μm, more typically <500 nm.